Towbar wiring

Jan 3, 2019
Being new to this malarky I am doing lots of homework before we get our first van and have just stumbled across a really interesting article that explains every aspect of van and trailer wiring including supplementary wiring, (i'll post the link in case anyone is interested or if links are allowed? )
Being a mechanic I have wired my fair share of standard 7 pin plugs and even the occasional euro 13 pin jobbies but never had anything to do with the supplementary plugs so wanted to see what they are about and what they are actually used for, (I fully intend to wire my own sockets into my car when i get round to it)
so with that in mind..I have a couple of questions..
Have any of you guys wired your own lighting and supplementary plugs in, and did anyone worry about uprating the alternator to cope with an extra load? ( I don't plan on doing this myself but wondered if anyone has had to do this as their cars alternator wasn't man enough?)
Mar 14, 2005
Hello Steve,

Welcome to the forum,

I have never heard of any caravanner having to upgrade their alternator (on a modern car) becasue of towing. As I understand it most alternators are only under maximum load for a relatively short period of time, so they usually have enough excess power to feed to the supplementary circuit. The circuit is usually controlled by a voltage sensing relay, and will only energise when there is excess alternator capability.

The caravan systems connected to the supplementary (Fridge, and battery charging) are quite happy to have an supply that has some off periods.
Sep 29, 2016
It's been many years since I heard of anyone fitting a higher output alternator for standard caravan purposes.

Not much other than road lights and the fridge and a charge to the leisure battery takes place, not much effort for modern alternators I understand.

Adding say a reversingrear view camera system would have little effect on your alternator capacity.

I can't see anything to be concerned about, I do not see the link you referred to, don't know if this link is of any use to you.
Just plug and play and enjoy :)
Jul 20, 2016
Just to back this up, my car has stop/start, and this still functions whilst towing which suggest the supply well outweighs the demand.
Nov 11, 2009
jamestaylor18 said:
Just to back this up, my car has stop/start, and this still functions whilst towing which suggest the supply well outweighs the demand.

My last Subaru and current Superb both have stop/start which is inhibited when connected to the caravan. Saves me having to turn it off anyway.
Jan 3, 2019
I don't have stop/start...but if i ever did happen to buy a car with it installed it would be the first thing to be disconnected :)
Jun 17, 2011
Don't forget that some modern cars have very light wiring and taht modern tow bar electrics use one mains feed and a set of sensors. The unit that controls the caravan electrics has a chip in it that senses what is needed and put that out. Fridge and van charging won't work unless the voltage is right which means generally only when the engine is running. The stop start system seems to have no effect.
Jan 13, 2019
I have been involved with lots of auto electrics over the years..The only thing I would add is ensure your battery is in good health. The majority of alternators will cope with the addition of towbar electrics. Saying that if your battery is close to the end of its serviceable life it will make the alternator work harder and can blow the Rectifier in the alternator.
Most reputable auto electricians will have a digital battery drop tester.
To give you an idea I just recently changed a 2 year old battery because of concerns with the cca (cold cranking amps) being a lot lower than what it should be. The drop tester picked it up which analyses the core of the battery.
Hope this helps.


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